Minneapolis's sculpture garden is great. And their science museum (k, that's actually in St. Paul) is really good--it has a staircase where each step plays a musical note when you walk on it! It's like that scene in BIG, with the giant keyboard, but better, because the giant keyboard is... a staircase. There's First Avenue, MIA and the Walker too.
Mall of America was more charming when there was Camp Snoopy and a giant inflatable Snoopy